Improving Pneumatic Energy Efficiency

Pat Phillips, AutomationDirect, Cumming, Ga.

Thu, 2014-01-16 14:02

Following some proven techniques can reduce pneumatic system energy costs by more than one-third.

Reducing energy consumption is a priority in most every manufacturing plant and industrial facility, as no company can afford to throw money away using machines and processes that waste energy. Because pneumatic systems are ubiquitous throughout manufacturing and can account for a large share of a plant’s power costs, it is extremely important that they run efficiently.

Unfortunately, many users have the mindset that pneumatic systems are inherently inefficient, and so overlook opportunities for energy savings. In addition, some manufacturers of industrial equipment and robots tend to focus on ensuring the pneumatic systems perform their intended functions, and in the process neglect efforts to reduce operating costs. These OEMS should instead recognize that plant operators are becoming more concerned with total cost of ownership (TCO), of which energy cost is a major component. These customers know that energy usage can account for up to 75% of machine and robot TCO, and they’re looking to suppliers to help them reduce that bill.

The old business model of only caring about performance and not about efficiency is dying. In the long run, OEMs that include energy efficiency as part of the overall performance of their pneumatic systems will be better positioned to succeed than those that neglect TCO.

Fortunately, both OEMs and users can improve the energy efficiency of pneumatic systems, with tactics that range from better engineering decisions in the design stage to adjustments and maintenance on existing systems.

According to data from the U. S. Department of Energy, manufacturers spend over $5 billion each year on energy for compressed-air systems. By optimizing these systems, companies can reduce their compressed-air energy consumption by anywhere from 20 to 35%. (The DOE offers guidelines for determining the cost of compressed air in a plant, as well as tips on how to reduce compressor energy consumption. Visit www.energystar.gov/buildings/sites/default/uploads/tools/compressed_air1.pdf for more information.)

Right-size components

Correctly sizing pneumatic-system components helps cut costs in several ways, as each component can affect other parts of the system. For example, undersized control valves may initially be cheaper than larger, right-sized units, but they require the air compressor to work harder to get the proper pressure to the actuators.

On the other hand, while some oversizing is necessary to compensate for pressure fluctuations and air losses, grossly oversized components account for one of the biggest energy drains in a pneumatic system. If an engineer simply oversizes from a 2 to 3-in. cylinder, for example, required air volume will more than double. Correctly sizing a cylinder can reduce its air consumption by at least 15%, which becomes even more significant in systems with many cylinders that cycle thousands of times over their operating life.

In general, most loads and speeds require only 25% additional capacity to ensure proper operation. While many calculations and considerations go into right-sizing components (such as whether a load is rolled or lifted), software packages, online calculators, and even iPhone apps can assist with computations. By spending a little more time in the design phase, OEMs can deliver substantial energy savings to their customers.

Right-sizing pneumatic components will not only increase customer satisfaction, it lets OEMs cut their own expenses. Larger and heavier components use more energy and create a larger footprint, which no manufacturer likes, and they cost more up front.

Optimizing pressure

As compressed air flows through typical circuits, air pressure drops due to changes in demand, line and valve-flow resistance, and other factors. But many of these losses are simply because the distance between the compressor or supply point and the actuator is longer than necessary.

Designs that use the shortest tubing possible can reduce energy consumption as well as cycle times. Typically, tubing between control valves and cylinders should be less than 10-ft long. Longer lengths require more pressure so that force, speed, and positioning capabilities aren’t compromised.

Another way to eliminate unnecessary consumption is ensuring actuators use only the pressure needed to perform a task. Sometimes, operators on the plant floor increase supply pressure in the belief that it improves performance. However, all this does is waste energy and money. Installing sensors that monitor pressure, and pressure regulators that maintain correct settings, can keep pressure within the minimum and maximum parameters.

Many engineers also design systems that deliver more pressure than needed to the actuator. Regulators that control pressure to individual pneumatic cylinders will increase energy efficiency, in many instances generating savings of up to 40%.

The same holds for complete machines. OEMs typically design standard equipment to accommodate users who need the highest forces. Adding pressure regulators lets OEMs more accurately size components while still meeting a range of performance requirements.

Don't overlook the return stroke

Another way to conserve energy is by supplying the correct pressure for an actuator’s return stroke. Most applications only move a load in one direction. However, many machines use the same pressure for both the working and return strokes.

For example, a material-handling system that pushes boxes from one conveyor to another needs high cylinder force only in one direction. The working stroke may demand 100 psi to move a box, but the low-force return stroke only requires 10psi. Using the same pressure in both directions wastes energy. Reducing the pressure on the return stroke saves 90% of the volume of compressed air. Because that conserves compressed air, a lot of energy is saved over the thousands of cycles that the action is performed.

Another important and often overlooked benefit of regulating air pressure to the minimum required level: It lessens wear and tear on the pneumatic and related components. Not overpressurizing the retract stroke reduces vibrations and shock to the machine. Moreover, adding a quick-exhaust valve can reduce cycle times because exhaust rate on the return stroke affects cylinder speed.

Processes with shorter strokes can use single-acting, spring-return cylinders. A control valve ports compressed air to the cylinder for the working part of the stroke, and then exhausts that air. During the return stroke the spring, or sometimes merely the weight of a mechanism, brings the cylinder back to the starting position.

A typical case where single-acting, spring-return cylinders can reduce energy demand involves presses. In this type of application, a cylinder pushes two items together such as a bearing into a housing, or a plug into a hole. The job demands a significant amount of force to press the parts together, but only a small amount to retract. This makes it a good candidate for energy savings by minimizing return-stroke air consumption.

Turn it off

Shutting down a machine when it’s not working seems like an obvious way to save energy. While some elements of a system, such as air bearings, can require pressure even when the machine is off, the required compressed airflow is usually much less than that needed during normal operations.

However, many installations have no automatic way to reduce or stop airflow to idle machines. Reduced staffing often means that manufacturers can no longer send maintenance workers to manually turn off air to specific machines. In these instances, automatic air-reduction controls will lower air pressure or, if appropriate, shut it off completely when the machine isn’t working, more than paying for itself in short order.

Minimize leaks

Leaks are common and expensive in pneumatics systems. Statistics from the U. S. Department of Energy show the average manufacturing plant loses 30 to 35% of its compressed air due to leakage. The good news is many leaks can be prevented or repaired.

There are many points between the compressor and the load where leaks can be fixed, with valves and seals two main areas for improvement. Deteriorated seals and certain valve designs, such as lapped-spool valves with metal seals, have inherent internal leakage that is constant as long as air is supplied to the valve. Switching to valves with soft seals can significantly lower this leakage.

However, it’s important to note that air consumption in lapped-spool and metal-sleeve valves doesn’t vary during operation. On the other hand, during an open crossover when the valve shifts, a soft seal produces hundreds of times more leakage than a lapped spool-and-sleeve valve. Therefore, selecting the right type of valve for an application can minimize air leakage.

It’s equally important to consider environmental conditions such as temperature and humidity, and type or lack of lubrication, as these all affect the leakage rate of a seal. In some instances, hardy and relatively expensive seals like Viton, Teflon, or polyurethane may be the best option.

Systems approach

Pneumatic systems aren’t quite as simple as they might first appear. The engineering concept of actuating valves and moving loads with air is quite straightforward, but optimizing pneumatic-system designs and maintenance involves many variables.

While operating conditions and component selection are large factors in the general inefficiency of these circuits, pneumatic systems can be greatly improved by implementing the concepts discussed here. OEMs play a big part because much of the energy inefficiency of pneumatic systems can be remedied at the design level. Machine users also have a crucial role to play as they are responsible for the overall operation and maintenance of a plant’s pneumatic system.

In today’s world, users are more aware of how energy consumption affects their bottom line. As such, OEMs must consider their customers’ TCO, not just upfront costs.